In a typical VoIP (Voice over Internet Protocol) system, a PBX (private branch exchange) interfaces between devices (e.g. IP (Internet Protocol) telephones, video homes, wireless devices, etc.) on a network and an interface with a PSTN (public switched telephone network). The PBX acts as a single receiving point for all devices on the network for invitations for media sessions (e.g. incoming calls). The PBX then routes these incoming invitations to devices on the network to which the invitations are addressed. All outgoing media session invitations from the devices may also be routed through the PBX who will handle any authentication requests from the interface before the outgoing media session invitations are accepted for forwarding to the PSTN. The PBX is the single point of connectivity between the interface and the devices on the network.
In a peer-to-peer (P2P) network there is no PBX or other similar device to route invitations for media sessions and act as the single point of connectivity. However, many interfaces expect and/or require such a single point of connectivity.